1. Field of the Invention
The present invention relates to the production of concentrated aqueous solutions of bromine-based disinfectants. More particularly, the present invention relates to a process for the generation of hypobromous acid.
2. Description of the Prior Art
Industrial process waters, such as cooling tower water, paper plant water, and the like are often contaminated with undesirable microorganisms. Waste waters from sewage plants constitute another source of contaminants such as coliform, viruses, and other organisms which must be eliminated before discharge.
In cases where treated water is discharged into rivers and streams, it is important that low residual oxidant be maintained and that any disinfectant discharged be of low toxicity.
Chlorine has long been employed in the disinfection of drinking water, process water, cooling water, waste water, and the like. However, the use of chlorine has generated problems in some of the above uses. For example, the use of chlorine in the disinfection of drinking water containing organic contaminants has led to the formation of high levels of trihalomethanes, which are carcinogenic. The use of chlorine in process and cooling water has led to excessive corrosion of equipment. In addition, chlorine residuals in waste water lead to destruction of aquatic life when the waste water is discharged into streams or rivers.
Substitution of bromine-based disinfectants, such as hypobromous acid (HOBr), for chlorine has the following advantages:
(i) hypobromous acid it is a more powerful disinfectant than chlorine, allowing the maintenance of much lower residuals to attain the same kill; PA1 (ii) hypobromous acid decomposes into harmless products much more rapidly than chlorine-based disinfectants such as hypochlorous acid, giving a much lower concentration of toxic materials at discharge into marine waterways; PA1 (iii) hypobromous acid is much less toxic to aquatic life (especially the initial species in the aquatic food chain) than the same concentration of hypochlorous acid; PA1 (iv) the ability to use lower residual concentrations of hypobromous acid, leads to lower concentrations of harmful by-products such as trihalomethanes; and PA1 (v) lower residual concentrations of hypobromous acid, plus its rapid rate of decomposition to non-toxic products, reduces the cost of total dehalogenation (before discharge) to insignificance. PA1 (a) separately feeding compounds, which when reacted together yield hypobromous acid, via separate feed lines into a water stream, followed by complete mixing of the compounds; PA1 (b) allowing the compounds to react in the water stream to yield hypobromous acid and thereby form the aqueous hypobromous acid solution, and; PA1 (c) monitoring the pH of the formed aqueous hypobromous acid solution using a pH meter provided downstream of the reaction between the compounds which yield the hypobromous acid. PA1 (a) prereacting compounds which when reacted together yield hypobromous acid; PA1 (b) feeding the reaction mixture into a water stream to form the aqueous hypobromous acid solution; and PA1 (c) monitoring the pH of the hypobromous acid solution formed using a pH meter provided downstream of where the reaction between the compounds which form the hypobromous acid occurs.
Several paths of decomposition of hypobromous acid are possible as shown in the following reactions: EQU 2HOBr.fwdarw.2H.sup.+ +2Br.sup.- +O.sub.2 .uparw. EQU 3HOBr.fwdarw.BrO.sub.3.sup.- +2Br.sup.- +3H.sup.+ EQU 5HOBr.fwdarw.H.sup.+ +BrO.sub.3.sup.- +2Br.sub.2 +2H.sub.2 O
There is much less corrosion with bromine-based disinfectants, such as hypobromous acid, than with chlorine. This is true for two reasons. First, the residual concentration of hypobromous acid is much lower than chlorine due to its higher potency against organisms and its rapid decomposition rate. Secondly, it has a lower oxidation potential than chlorine which has 2-3 times more corrosive effect than bromine-based disinfectants.
Various methods have been employed to introduce bromine-based disinfectant systems into aqueous solutions for the purpose of preventing the growth of harmful and offensive microorganisms.
Among these are various N-brominated organic compounds, in which the bromine atom is loosely bound and capable of rapid reaction to give hypobromous acid when added to water. An example of this type of compound is bromochlorodimethylhydantoin (BCDMH) which, when added to water, undergoes hydrolysis to give hypobromous acid and hypochlorous acid by the following reaction: EQU C.sub.5 H.sub.6 BrClN.sub.2 O.sub.2 +2H.sub.2 O.fwdarw.C.sub.5 H.sub.6 N.sub.2 O.sub.2 +HOBr+HOCl
The hypohalous acids (HOBr and HOCl) thus generated are powerful disinfectants and will kill any organisms present and prevent the growth of any objectionable microbiological life forms. However, this type of brominated organic compound has several disadvantages. The cost is very high and a dimethylhydantoin by-product is produced. The presence of hypochlorous acid in the solution poses problems on discharge into streams or rivers due to its stability and toxicity to marine life.
Another method of generating hypobromous acid is by the hydrolysis of bromine chloride (BrCl) according to the following reaction: EQU BrCl+H.sub.2 O.fwdarw.HOBr+HCl
Bromine chloride is a liquid under pressure. When used in the same type of cylinders used for chlorine, it requires a specially designed, expensive liquid feeder to proportion the bromine chloride into water. It also creates safety concerns due to the necessity of storing large quantities of gas on-site.
Yet another method of generating hypobromous acid is to chlorinate water and then to pump a solution of sodium bromide (NaBr) into water containing hypochlorous acid (HOCl). The following equations illustrate this method: EQU Cl.sub.2 +H.sub.2 O.fwdarw.HOCl+HCl EQU HOCl+NaBr.fwdarw.HOBr+HCl
The disadvantage in using this method is that the water contacts chlorine before the hypobromous acid is formed and chlorinated by-products may be formed in waste the water. Also, in high chlorine demand water, the chlorine may not be available to oxidize the bromide ion (Br--) to hypobromite ion (OBr--) due to prior reaction. In addition, there is no control of reaction conditions to assure optimum conversion of the bromide to hypobromous acid. The concentration, pH and bromide/chlorine (Br/Cl) mole ratio are all important in the above reactions. It is almost impossible to control all of these variables using a pump to add the bromide ion to the chlorinated water.
U.S. Pat. No. 4,451,376 (to J. D. Sharp) indicates that it is not efficient to prepare hyprobromous acid directly in a biologically contaminated water system due to the loss of hypochlorous acid and the resulting poor conversion of bromide ion to hyprobromous acid. It also points out that the hyprobromous acid should be prepared as a treating solution which then contacts the unwanted microorganism. It further points out the fact that if sodium hypochlorite or chlorine is combined with a bromide salt in a mixing tank in an effort to produce hypobromous acid, the hypobromous acid is not stable at concentrated levels and decomposes to bromine, bromate and bromite.
Thus, there is no method for generating hypobromous acid wherein all of the variables can be controlled by a single apparatus and where the yield of hypobromous acid approaches 100%.
Accordingly, it is an object of the present invention to provide novel processes for the generation of relatively concentrated solutions of hypobromous acid. Another object of the present invention is to provide a process which gives high yields of relatively concentrated hypobromous acid in water from non-hazardous reactants.
These and other objects, as well as the scope, nature and utilization of the invention will become apparent to those skilled in the art from the following description, the drawing, and the appended claims.